In recent years and continuing, among plasma processing apparatuses, the microwave plasma processing apparatus is attracting attention. Compared with other plasma processing apparatuses, such as a parallel plate plasma processing apparatus and an ECR plasma processing apparatus, the plasma potential is relatively low in the microwave plasma processing apparatus, enabling generation of plasma having a low electron temperature and low ion energy.
Therefore, with the microwave plasma processing apparatus, it is possible to prevent metal contamination and ion irradiation damage to a substrate subjected to plasma processing. In addition, because a plasma excitation space can be separated from the plasma processing space, it is possible to carry out plasma processing independent of materials of the substrate and patterns formed on the substrate.
In the microwave plasma processing apparatus, a gas for exciting plasma (referred to “plasma excitation gas”) is fed into a chamber first, and then the microwave is introduced to the plasma excitation gas to start the generation of plasma (plasma ignition). Because of the high frequency of the microwave, the electrical field of the microwave changes before electrons of the plasma excitation gas are sufficiently accelerated, making plasma ignition difficult. Additionally, in recent plasma processing, a pressure of the plasma excitation gas as low as, for example, 67 Pa (about 0.5 Torr) is sometimes required. Because of the low pressure, the density of the plasma excitation gas is low, further making plasma ignition difficult.
In the microwave plasma processing apparatus, differing from the parallel plate plasma processing apparatus, the microwave is emitted from a microwave antenna, and there is no electrical field applied to the substrate to be processed, therefore, free electron emission or other phenomena causing plasma ignition do not occur, making plasma ignition more difficult.
In the microwave plasma processing apparatus of the related art, usually, the pressure inside the processing chamber is set high at the time of plasma ignition, for example, the pressure is set to 133 Pa (about 1 Torr) so as to induce plasma ignition easily; after the ignition, the pressure is lowered to, for example, 7 Pa (about 50 mTorr). However, in this plasma ignition, an additional control procedure not relevant to the object plasma processing has to be gone through to increase the pressure inside the processing chamber for plasma ignition and to lower the pressure after the plasma ignition, and this lengthens the preparation time before plasma processing starts, and lowers throughput.